Both of these joined cases involved the right or otherwise to pension benefits and the particular dates from which entitlements might apply. O'Brien was a part-time judge and sought pension entitlements pro-rated with full-time judges. In this particular case, the claimant sought back-dating of pension entitlements to the date he started work (1978) rather than 2000, when the Part Time Workers Directive was transposed into UK law. The Court of Appeal ruled against him - the principle of 'no retro-activity' means that EU laws should not have a retrospective effect; they should apply from the date on which they are required to be law. In his case that was 2000, so earnings before that could not be counted towards pension entitlements.

The retro-active principle was also applied to Mr Walker, the second claimant, a gay man who was seeking a survivor pension to apply to his male civil partner (later spouse) under a company pension scheme that allowed a survivor's pension to be paid in the case of heterosexual couples. Unfortunately for Mr Walker, the Equal Treatment Framework Directive that protected same-sex couples was introduced in 2003, after he had retired, so there were no pension contributions after 2003 to look at. And his partner could only be treated as a spouse after 5 December 2015, when the Civil Partnership Act 2004 was brought into law in the UK.

The second principle applied in these cases is that amending legislation applies, unless otherwise specifically provided, immediately to the future effects of a situation which arose under the law as it stood before amendment. This principle applies only to EU legislation and rules. It does not apply to judge-made law, although there are different techniques by which the Court of Justice is able to avoid practical retro-activity. In the case of Mr Walker, he had left employment after the Directive had been transposed into UK law - the contractual pension entitlement was permanently fixed and there was nothing for the legislation to impact on in future.
http://www.bailii.org/ew/cases/EWCA/Civ/2015/1000.html

NOTE: Mr Walker has been granted leave to appeal to the UK Supreme Court, according to an article in pensionsage.com on 11 August 2016:

http://www.pensionsage.com/pa/Same-sex-couple-to-appeal-landmark-pensions-case-in-Supreme-Court.php?

The O'Brien appeal is listed on the Supreme Court's website:

https://www.supremecourt.uk/cases/uksc-2015-0248.html